1. Field of the Invention
The present invention relates to an apparatus for controlling a three-phase AC (alternating current) motor on a two-phase modulation technique.
2. Description of the Related Art
In various applications of electronic motors, a higher need for reducing loss caused in the motors has arisen, partly due to environmental problems. This is also true of a PWM (pulse width modulation) inverter to operate a three-phase AC motor.
The PWM control for the three-phase AC motor is usually based on a three-phase modulation technique. However, one recent trend for the PWM control is to use a two-phase modulation technique which can be divided into two types.
One type of two-phase modulation technique is called “π/3-fixing type” (which is sometimes called “fixed-phase 60-degree switchover type”), which is taught by some references. The two-phase modulation technique employs the fact that a motor current depends on phase-to-phase voltage, not on the phase voltages. Hence, the inverter is driven such that, with the phase-to-phase voltages kept at a predetermined voltage, switching elements assigned to each phase are switched “on” for a predetermined period of time so as to fix its phase voltage at a high power-supply voltage level or a low power supply-voltage level during a period of time corresponding to an electrical angle of π/3 (60 degrees). Fixing the voltage at the predetermined power-supply voltage level is performed in sequence for each of the three phases U, V and W, so that the loss caused due to switching in the converter (switching loss) can be reduced.
The other type of two-phase modulation technique is called “2π/3-fixing type” (which is sometimes called “fixed-phase 120-degree switchover type”), which is taught by, for example, Japanese Patent Publication No. 2577738 and Japanese Patent Laid-open Publication No. 11-262269. In those publications, each phase voltage is fixed at a high or low power-supply voltage level for a predetermined interval of time corresponding to an electrical angle of 2π/3 (120 degrees) and this phase control is carried out for each of the phases U, V and W in sequence to reduce the switching loss in the inverter.
An additional configuration in the foregoing patent publication is to stop operating the motor on the two-phase modulation and, instead, apply three-phase voltages to the motor, if the amplitude of each phase voltage is smaller.
The above motor is very frequently used as an electric motor for running an electric vehicle, hybrid vehicle, and others. Such motors are necessary to operate in all the four quadrants of a two-dimensional motor-output coordinate defined by the two axes assigned to torque and the number of rotations of the motor. In terms of this requirement, the foregoing “2π/3-fixing type” is insufficient, because the switching loss cannot be fully suppressed in some operation ranges showing higher speeds and larger amounts of torque.
In the case of the foregoing “π/3-fixing type,” a fixing period of time at each fixed phase and positive and negative peaks of phase current flowing each phase can be synchronized with each other. Compared to “2π/3-fixing type,” a higher suppression of the switching loss is therefore gained. However, it has been pointed out that, when the output voltage is smaller in amplitude, switchover timing for the fixed phases tends to be erroneous, thus failing to keep a well-controlled two-phase-modulation operation of the motor.
In addition, the foregoing switchover between the three-phase modulation drive and the two-phase modulation drive on “2π/3-fixing type” do not always operate well. This is attributable to the fact that the three-phase voltages provide time-dependent AC waveforms which are likely to undergo mixing with noise voltage so that precision in switchover timing becomes poor. In other words, the switchover that relies upon detection of the amplitudes of the three-phase voltages has a difficulty in sufficiently reducing the switching loss.
Meanwhile, in mounting a large-sized motor, such as three-phase AC motor for running a running a vehicle, into a vehicle engine room, it is extremely strictly important to suppress the temperature of the switching arm elements of the inverter.